1. Field of the Invention
The present invention relates to a tunable multimode wavelength division multiplex Raman pump and amplifier, and a system, method, and computer program product for controlling the same.
2. Discussion of the Background
With the explosion of the information age has come a demand for larger data transmission capacity for optical communication systems. Conventionally, optical communication systems transmitted data on a single optical fiber using a single wavelength of light (e.g., 1310 nm or 1550 nm). Signals at these wavelengths were desirable since they have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of these single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route which greatly increased the cost of optical fiber networks.
To mitigate this problem, wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system have become desirable. In a WDM system, a plurality of optical signals, each having a different wavelength, can be transmitted simultaneously through a single optical fiber.
Optical fiber communication systems transmit optical signals over considerable distances. However, the signal strength of the optical signals attenuates with distance because of absorption and scattering. Signal strength attenuation ultimately results in signal reception degradation if the signal strength is not kept above background noise (or other sources of noise) by a predetermined amount. Amplifiers are used to keep the signal strength above background noise by a predetermined amount. In general, there are two approaches to amplifying an optical signal: the first, is to use an electronic repeater, which converts the optical signal into an electric signal, amplifies the electrical signal, and then converts the amplified electrical signal back into an optical signal for further transmission along an optical fiber; the second, is to amplify the optical signal itself. Two types of amplifiers that can be used to amplify an optical signal according to the second approach are rare earth doped fiber amplifiers such as erbium doped fiber amplifiers (EDFA), and Raman amplifiers.
EDFAs are currently the most widely used optical amplifiers for WDM systems and are effective and reliable for optically amplifying WDM signals. However, EDFAs have an amplification bandwidth that is limited in range, and produce a wavelength-dependent gain profile. These two characteristics of EDFAs are undesirable for WDM signals, which are spectrally distributed, since a non-uniform amount of gain will be applied to the various WDM channels, depending on the wavelength of the channels. To offset this effect, a gain flattening filter may be used to obtain a uniform or flat gain profile (having a gain deviation of less than 1 dB) across the entire communication band. The gain flattening filter is designed to have a loss profile having a shape that is the inverse of the shape of the gain profile. Gain flattening filters, however, are limited to a particular gain profile, and are not dynamically adjustable to compensate for changes in a magnitude of the gain of the EDFA. Therefore, a flat gain profile cannot be maintained when the gain of the EDFA is changed, or if the attributes of the communications network are changed, such as by adding more WDM signals. In addition, the gain flattening filter decreases the total amount of power launched into an optical fiber.
Raman amplifiers use a phenomenon known as Stimulated Raman Scattering (SRS) of light within an optical fiber to achieve a gain in a particular wavelength band. The inelastic scattering process generates an optical phonon and a co-propagating Stokes wave, light that is downshifted in frequency from the pump light by an amount equal to the phonon frequency (i.e. total energy is conserved). In silica fibers, the peak SRS gain occurs at about 13 THz below the pump light frequency (or conversely, at a wavelength that is longer than a wavelength of the light pumped into the optical fiber by about 100 nm). Since Raman amplification is a scattering process, unassociated with the resonance properties of any particular material, one can generate a Raman gain spectrum for pump light at any wavelength. Therefore, changing a wavelength of the pump light, changes the wavelength at which a peak gain is applied to WDM signals, thereby amplifying some WDM signals more than others. By multiplexing several different pump wavelengths into the same fiber, one can generate a reasonably flat gain spectrum over an arbitrary bandwidth. Because Raman amplifiers require a greater pumping power to obtain the same gain as an EDFA, Raman amplifiers have primarily been used in signal wavelength bands outside of the amplification bandwidth of EDFAs.
Although a Raman amplifier amplifies a signal over a wide wavelength band, the gain of a Raman amplifier is relatively small and, therefore, it is preferable to use a high output laser device as a pumping source. However, increasing the output power of a single mode (or frequency) pumping source beyond a certain threshold leads to undesirable stimulated Brillouin scattering and increased noise at high peak power values. As recognized by the present inventors, to prevent this problem, a multimode laser device is preferably used as a pumping source in a Raman amplifier. A multimode laser has a plurality of oscillating longitudinal modes, each providing output power at less than the threshold at which stimulated Brillouin occurs. A multimode laser can provide a sufficient amount of output power to achieve Raman amplification distributed over the various modes (i.e., wavelengths of output light), as opposed to providing the power all at a single wavelength.
To control the wavelength of the light emitted from the pumping source, and therefore, determine what wavelength of signal will be amplified, it is well known to use fiber gratings. A fiber grating selectively reflects certain wavelengths of light causing a laser beam of a specific wavelength to be output. Fiber gratings are known to be included in the core of an optical fiber, separate from the laser device itself. However, having the grating separate from the semiconductor laser device has been found to be problematic in that it allows for noise to be introduced, for instabilities due to the mechanical vibrations that can occur between the semiconductor laser device and the optical fiber including the fiber grating, and for losses.
However, as described in U.S. patent application Ser. No. 09/832,885, a diffraction grating may be included within a spacer layer of the semiconductor device itself. By having the semiconductor laser device itself control the output characteristics of the generated light, without the use of an external grating, opportunities for noise, instabilities, and losses are minimized. The diffraction grating is configurable, allowing for the wavelengths and spacing between the multiple modes of light being generated to be predetermined.
As described in Bruce, E. “Tunable Lasers,” IEEE Spectrum, pages 35–39, February 2002, there are a variety of other types of tunable lasers made for use in WDM systems, although the primary application is for generating a WDM signal at a particular frequency (or wavelength). As recognized by the present inventors, since none of these tunable lasers are made for operation as a Raman pump source that intentionally outputs light at more than one frequency, it is unclear from the literature how, or whether, these devices could be adapted for use in multimode applications.
As described in U.S. Pat. No. 6,292,288, in order to achieve a uniform gain profile over a broad range of wavelengths, a Raman amplifier can include multiple pump lasers, each providing multimode light having a predetermined spectral width, centered at a different central wavelength. By properly spacing in wavelength the pump lasers with predetermined optical output levels, it is possible to achieve a composite gain profile that is flat over a broad range of wavelengths, and therefore to provide Raman amplification over a broad range of wavelengths.
U.S. patent application Ser. No. 09/775,632 describes a system through which Raman amplification performance can be controlled. As described in that application, by controlling an output power of each of the lasers of a particular Raman amplifier, the desired gain characteristics can be maintained. Moreover, by monitoring and controlling a portion of a network, the Raman amplification performance of that portion can be controlled through cooperative adjustments made to one or more of the Raman amplifiers (or of individual pump lasers of a particular Raman amplifier) that impact that portion of the network.
FIG. 1 is a block diagram of a conventional Raman amplifier 100. The Raman amplifier 100 includes an amplifier fiber (optical fiber) 103, a WDM coupler 104, a pumping device 107, a control unit 119, and optional polarization independent isolators 102, 105. The Raman amplifier 100 is connected (or merely coupled) to an input fiber 101 and an output fiber 106, which may be optical transmission fibers such as single mode fibers (SMF), dispersion compensation fibers (DCF), dispersion flattening fibers, etc. The amplifier fiber (or optical signal transmission fiber) 103 may be similar types of fibers as well.
The Raman amplifier 100 is connected to a network 122 via a communication link 123. The network 122 is also connected to other amplifiers 124, 125 as well as a remote device controller 121. The remote device controller 121 monitors the operational status of the Raman amplifier 100 as well as the other amplifiers 124, 125. The network 122 may be a proprietary wireless or wired network, or another network that is publicly accessible, such as the Internet or a hybrid network, part proprietary and part publicly accessible. While the Raman amplifier 100 may operate autonomously, it may receive additional information about the overall system performance, such that the control unit 119 can adapt the amplification performance of the Raman amplifier 100 to help offset any adverse affects to the system's performance, as might be necessitated by a change in conditions, described in the additional information. As an example, this additional information may be that a replacement fiber with different attenuation characteristics is being used to interconnect two cascaded Raman amplifiers in a WDM system. In this case, the Raman amplifier 100 may set a new “target” amplification performance so as to normalize the channel characteristics for all of the WDM channels, despite the fact that the new fiber may attenuate some of the channels by a lesser amount than others.
The pumping device 107 includes Fabry-Perot type semiconductor lasers 109, 110, 111, 112, wavelength stabilizing fiber gratings 113, 114, 115, 116, polarization couplers 117, 118, and a WDM coupler 108. The central wavelengths of the semiconductor lasers 109 and 110 and wavelengths of the fiber gratings 113 and 114 are the same wavelength λ1, and the central wavelengths of the semiconductor lasers 111 and 112 and reflection wavelengths of the fiber gratings 115 and 116 are the same wavelength λ2. The central wavelengths of the semiconductor lasers 109, 110, and 111 and 112 are respectively stabilized to λ1 and λ2 via the wavelength stabilizing fiber gratings 113, 114 and 115, 116.
Multimode light generated by the semiconductor lasers 109, 110 and 111, 112 is combined by polarization combiners 117, 118 for each central wavelength λ1 and λ2, respectively. The light output from the polarization combiners 117, 118 is combined by the WDM coupler 108. Polarization maintaining fibers 126 are used in the connections between the semiconductor lasers 109, 110, 111, 112 and the polarization combiners 117, 118 to maintain two different polarization planes. This ensures that an input signal to the Raman amplifier 100 will be adequately amplified regardless of its orientation in the signal fiber 101 or amplification fiber 103.
The pumping device 107 in this example includes two pumps that provide light having two different wavelengths λ1 and λ2 to the amplifier fiber 103 (i.e., a first pump that provides light having a central wavelength of λ1, and a second pump that provides light having a central wavelength of λ2). Further, as noted in U.S. Pat. No. 6,292,288, a wavelength interval between the wavelengths λ1 and λ2 is selected to be in a range of 6 nm to 35 m in order to provide a flat gain profile over a range including both λ1 and λ2.
The light output from the pumping device 107 is coupled to the amplifier fiber 103 via the WDM coupler 104. An optical signal (e.g., a WDM signal) is incident on the amplifier fiber 103 via the input fiber 101. The optical signal is then boosted in signal level after the gain medium has being excited by the light pumped into the amplifier fiber 103, the net result being that the optical signal is Raman-amplified. In addition, the Raman-amplified optical signal is passed through the WDM coupler 104 and is transmitted toward the control unit 119, where a part of the amplified optical signal is branched to form a monitor signal (or sampled output signal), while the majority of the signal is output on the output fiber 106.
The control unit 119 includes a processor to assert control over the amplification performance of the Raman amplifier. The control can be based on either the monitored signal or an external source, such as, for example, a control signal received from the remote device controller 121. The control unit 119 generates a control signal on a bus 120, that includes a sufficient number of control lines, so as to allow for control of the drive currents and for the individual semiconductor lasers 109, 110, 111, 112 to achieve a small gain deviation relative to a target gain profile (e.g., a flat amplification profile).
FIG. 1B is a block diagram showing a redundant pump source that is an alternative to each of the separate LD lasers 109, 110, 111, and 112. In particular, the redundant pump source of FIG. 1B includes LD pump source A and LD pump source B, each being multimode and having a same central wavelength. The outputs from the two different LD pump sources are combined in a 3 dB coupler and output as two output light beams, each of which includes half powers from each of the LD pump sources. The redundant pump sources are used because it is possible that one of the LD pump sources will fail. In this failure situation, the output power will be reduced by half, unless the input power to the LD pump module that continues to operate is increased to offset the optical power lost by the failure of the companion LD pump source. In this way, even if one of the LD pump sources fails, the failed light source is survived by its companion, which can still produce some pump power output.
As compared with EDFAs, for example, Raman amplifiers are more complex devices since they contain more laser diode modules, operate over wider bandwidths that are determined by system parameters, and require controllers that are able to establish predetermined amounts of gain across the amplification bandwidth, consistent with network requirements. As recognized by the present inventors, part of the complexity is manifested in a controller that is able to adjust pump output levels when environmental or network requirements change. For example, the central wavelengths of the pump modules will change as a function of temperature. This change in central wavelength will result in change in gain shape, which must be detected by the controller and compensated. However, changing pump power level also affects the Raman amplifier's gain characteristic, and thus optimum control is not always possible due to temperature induced wavelength shifts in pump light.
Likewise, changes in system requirements may create a situation where the amplification bandwidth of the amplifier must be changed (i.e., widen, or shift to another band). While some changes are possible by switching-in or switching-out pumps to accommodate bandwidth changes, this leads to more expensive amplifiers, because more pumps are needed in the amplifier, albeit not used until needed. Likewise, some of the pumps may degrade overtime or fail. On-board spares may be used to mitigate reliability concerns, however this solution is expensive to implement if all pumps are provided with an on-board spare.
The addition of channels within the amplification bandwidth is another possible scenario. In such a scenario, it may be necessary to increase amplifier power to avoid pump depletion. Since Raman scattering is a non-linear process, the amplifier gain cannot be increased by simply increasing the laser pump power, since doing so will change the power partitioning in the output wavelengths, which will likely result in a non-flat gain spectrum.
The present inventors have also recognized that while Raman amplifiers may be used in tandem with erbium doped fiber amplifiers (EDFA), the combination sometimes can result in poor performance of the EDFA. In this situation, inherent bandwidth or gain profile of an EDFA-based system may be suboptimal due to temperature induced gain profile changes in the EDFA and/or operator-changed EDFA gain.
The present inventors have also recognized that conventional Raman amplifier systems are “custom built” to address the specific requirements for system integrators. However, as with most manufacturing processes that require customization, the cost for producing custom product is substantially greater than that for mass produced devices. Thus the present inventors have recognized a need for a narrow band amplifier that is able to be “field-configured” so that the amplifiers may be made in bulk production, yet still used in a variety of different situations in operational systems.
The present inventors have also recognized that because conventional WDM pumped Raman amplifier systems have fixed pumping wavelengths, there are fewer degrees of freedom for making adjustments to Raman gain profiles when performing adaptive dynamic gain equalization. However, the present inventors recognized that by shifting frequencies (or wavelengths) of the pump sources themselves, provides yet another control variable that simplifies adaptive dynamic gain equalization and refines the ability to compensate for wavelength ripple and thus flattening composite gain profiles in cascaded optical amplifier systems.
The present inventors have also recognized that because conventional Raman amplifiers have fixed wavelength LD pumps, there is no possibility for providing an automated process for adjusting gain profiles through a process that includes wavelength shifting. Thus, the present inventors have recognized the possibility of using a tunable LD pump, in combination with a controller feedback mechanism, to provide a fully automated Raman amplifying system that uses a plurality of tunable Raman amplifiers (or alternatively just one tunable Raman amplifier).